Simple thread-safe cache spending minimum time in lock (thanks to Lazy).

ThreadSafeCache.cs

public class ThreadSafeCache<TKey, TItem>
{
    private readonly ConcurrentDictionary<TKey, Lazy<TItem>> _cache = new ConcurrentDictionary<TKey, Lazy<TItem>>();
    private readonly object _lock = new object();

    public TItem GetOrAdd(TKey key, Func<TKey, TItem> itemFactory)
    {
        Lazy<TItem> item;

        if (!_cache.TryGetValue(key, out item))
        {
            lock (_lock)
            {
                item = _cache.GetOrAdd(key, x => new Lazy<TItem>(() => itemFactory(x)));
            }
        }

        return item.Value;
    }
}